1. Field of the Invention
The present invention relates to an optical pickup device for use in an optical reproducing apparatus for reproducing information from a playback-only optical disk such as compact disk or laser disk, or in an optical recording/reproducing apparatus for recording/reproducing information on/from a write once, read many or rewritable optical disk.
2. Description of the Background Art
Generally, light intensity profile on the exit pupil of an objective lens for focusing a light beam onto a recording surface of a recording medium largely affects a spot size to be focused, and has a great influence on the performance of the optical pickup device.
It is known that the spot size being formed relies on a numerical aperture (NA) of the objective lens, oscillation wavelength xcex of light, and lens rim intensity (xe2x80x9cRimxe2x80x9d, a ratio of the rim intensity with respect to the center intensity). When the radius of the condensed beam having the center intensity of at least 1/e2 is represented as xe2x80x9crxe2x80x9d, if Rim=1.0 (i.e., the lens incident intensity is uniform through the surface), the radius xe2x80x9crxe2x80x9d can be expressed as the following expression (1):
r=0.41xcex/NAxe2x80x83xe2x80x83(1).
Thus, in order to reduce the beam spot size, it is necessary to decrease the oscillation wavelength xcex of the light and to increase NA of the objective lens.
Further, the expression (1) above holds only when Rim=1. It is well known that the beam spot size will become larger than in expression (1) when Rim becomes less than 1.0.
Thus, in order to reduce the beam spot size, it is considered that it is preferable to increase the lens rim intensity (Rim) so as to achieve uniform intensity distribution of the lens incident light down to the periphery of the lens.
In the optical pickup, it is also preferable that the light irradiated from a light source, e.g., a laser diode (LD), is coupled to the objective lens with the least possible loss.
If the coupling efficiency is increased, however, the lens rim intensity (Rim) is decreased, resulting in the light intensity profile as shown in FIG. 5(d). Referring to FIG. 5(e) illustrating the light intensity profile on the exit pupil at this time, a ratio of the light intensity at the peripheral portion with respect to that at the central portion within the diameter of clear aperture is decreased. Thus, as shown in FIG. 5(f), the light spot being condensed on the recording surface of the recording medium has a large diameter, which causes modulation transfer function (MTF) to be degraded in the high-frequency region. It means that resolution of a level required for reproduction cannot be obtained.
Conventionally, in order to prevent degradation of the resolution, an optical system is configured to have a relatively large lens rim intensity (Rim). Thus, although the coupling efficiency is lowered to some extent as shown in FIG. 5(a), the light intensity profile on the exit pupil becomes approximately uniform, as shown in FIG. 5(b), since the light intensity ratio of the peripheral portion with respect to the center portion within the diameter of the pupil is increased. Accordingly, the light spot size being focused onto the recording surface of the recording medium becomes small, as shown in FIG. 5(c), and therefore, the resolution of a level required for reproduction is achieved.
As described above, in the optical pickup, the light intensity profile within the diameter of the pupil can be made relatively even by setting the coupling efficiency as low as possible and increasing the lens rim intensity (Rim). This allows optimization of the light spot on the recording surface of the recording medium.
However, in the optical system above, when the polarized direction of the light incident on the disk is taken into account, polarized light (S-polarized light) that is perpendicular to the disk incident plane is more likely to generate reflected light called Fresnel reflection as the angle of incidence to the disk increases. This causes loss of the light to be transmitted into the disk.
In particular, when the light focused with the objective lens is being incident on the disk, the light from a portion closer to the periphery of the lens has a larger angle of incidence to the disk. The light from the outermost peripheral portion exhibits an angle of incidence corresponding to NA of the lens (Sinxe2x88x921 (NA)).
Thus, the loss of the light to be transmitted to the disk is increased as it is closer to the periphery of the lens, and the resultant Rim becomes smaller.
Conventionally, the disk surface is not provided with anti-reflection coating or the like considering cost and other factors. Thus, the light from the objective lens is likely to reflect on the disk surface, with its reflectance being varied dependent on the polarized direction of the light.
In general, it is known that the reflectance of a glass material to which no anti-reflection coating is applied varies dependent on the angle of incidence and the direction of polarization of the light. A relation between the reflectance and the angle of incidence when light with oscillation wavelength of 655 nm enters into a disk substrate having refractive index of n=1.51 is shown in FIG. 7.
As seen from FIG. 7, the reflectance increases as the angle of incidence increases. Further, the S-polarized light with respect to the disk substrate exhibits larger reflectance than the P-polarized light.
FIG. 6 shows a specific structure of an optical pickup device for use in a magneto-optical disk recording/reproducing apparatus representing a conventional optical information recording/reproducing apparatus.
Collimator lens 5 in FIG. 6 converts the light beam with wavelength of 655 nm emitted from a semiconductor laser, or laser diode, 1 into a collimated beam. Objective lens 8 is a condensing lens having NA of 0.47, which focuses the light beam onto a recording surface of magneto-optical disk 9.
The operation of a conventional optical information recording/reproducing apparatus will now be described with reference to the structure above. The light beam emitted from semiconductor laser 1 is turned into a collimated beam by collimator lens 5, and then focused with objective lens 8 onto the recording surface of magneto-optical disk 9. At this time, in the optical pickup, the coupling efficiency is set to the lowest possible level and the lens rim intensity (Rim) is set to the greatest possible value to realize relatively uniform light intensity profile within the diameter of the pupil, such that the light spot on the recording surface of the recording medium is optimized.
The angle of incidence xcex8 at the time when the light emitted from objective lens 8 enters into optical disk 9 becomes greater as it is from the position closer to the periphery of the lens. If NA is 0.47, xcex8 becomes at most Sinxe2x88x921 (0.47)=28 degrees. In this case, as shown in FIG. 7, the reflection on the disk surface does not vary whether the light is the P-polarized light or the S-polarized light, and the reflectance is extremely small.
Thus, there is almost no loss of the light at the time of incidence on the disk, and the light can be condensed with the Rim almost as designed.
However, with a high-density magneto-optical disk recording/reproducing apparatus currently under development, it is attempted to obtain an even smaller spot size by increasing the NA of the objective lens and reducing the wavelength of the laser diode. Accordingly, application of an objective lens with NA of 0.65 or greater, for example, has been taken into account.
With the lens whose NA is 0.65, the angle of incidence xcex8 at the time when the light emitted from the objective lens enters into the disk would become at most Sinxe2x88x921 (0.65)=40.5 degrees. In this case, as seen from the graph of FIG. 7, the reflection on the disk surface greatly varies between the P-polarized light and the S-polarized light, and the reflectance of the S-polarized light becomes considerably large.
Thus, there is a large amount of loss of the S-polarized light at the time of incidence onto the disk, and Rim becomes smaller in the S-polarized direction. The resulting beam spot is thus elongated in the S-polarized direction.
As a result, despite the increase of NA of the objective lens, the condensed beam spot of a desired size cannot be formed within the optical disk. This hinders increase of the recording density and makes it difficult to obtain reproduced signals in good condition.
These problems are profound especially when an objective lens of high NA with an increased light incident angle is employed.
As a countermeasure against these problems, Japanese Patent Laying-Open No. 11-31337 discloses application of anti-reflection coating onto a disk surface. This however increases the cost, and also poses other problems. Specifically, as the angle of incidence of the light entering into the disk varies from 0 degree to Sinxe2x88x921 (NA) degrees, it is necessary to apply anti-reflection coating with extensive coverage, which makes fabrication of the coating as well as increase of yields thereof difficult.
The present invention is made to solve the above-described problems, and its object is to reduce the size of a beam-condensed spot on a recording medium by reducing the spot diameter in a desired direction (a radial direction or track direction when the recording medium is a disk), without forming anti-reflection coating on the surface of the recording medium, thereby enabling high-density recording in the relevant direction.
According to an aspect of the present invention, the optical pickup device includes light generating means and an objective lens for focusing light emitted from the light generating means onto a recording surface of a recording medium, wherein the oscillation direction of wavefront of the light incident on the objective lens is made equal to the radial direction of the recording medium.
According to another aspect of the present invention, the optical pickup device includes light generating means and an objective lens for collimating light emitted from the light generating means onto a recording surface of a recording medium, wherein the oscillation direction of wavefront of the light incident on the objective lens is made equal to the track direction of the recording medium. The track direction is a direction that intersects with the radial direction at right angles.
The numerical aperture (NA) of the object lens is preferably at least 0.65. The recording medium preferably has data formed of pits. The recording medium may be a phase change recording medium or may be a magneto-optical recording medium.
If intensity distribution of the light incident on the objective lens is in the form of an ellipse, the major axis direction of the ellipse is preferably equal to the oscillation direction mentioned above.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.